Depends on how they do it. Normally the idea is not to increase the credibility but it is to increase the marketing of it, you would want more and more people to be interested in the project you do and when you are giving money to people for signature campaigns you are getting a lot in return, people all over bitcointalk will be hearing about your project which will create this "demand" for your project, not a ton but at least people will hear about it and know about it even if they do not get involved and a bit of them will get involved as well.

It is not always a great profitable thing, you are not going to spend a thousand dollars on signature campaign and get more than a thousand dollars in return from funding, but it would be at least a brand awareness type of deal.